This invention relates to a method for treating manganese nodules occurring on the ocean floor, and particularly relates to a process for extracting and recovering valuable metals in manganese nodules.
In recent years, the utilization of manganese nodules has become of great importance. This is because manganese nodules contain, in addition to manganese which is a principal component, valuable metals such as nickel, cobalt and copper, though the content is as low as about 1% for each metal, these metals have industrial importance and are being exhausted on the terrestrial areas (for example, nickel ores from New Caledonia which has great nickel deposits in the world had a grade of 9% level at the beginning, but the grade has fallen to the order of 2% in recent years and, despite this, the consumption is increasing at a very high rate), and potential reserves of deep-sea manganese nodules are enormous (estimated quantities of the metals utilizable as resources are 16.times.10.sup.8 tons for Ni, 10.times.10.sup.8 tons for Co, 5.times.10.sup.8 tons for Cu and 700.times.10.sup.8 tons for Mn).
A typical example of the composition of manganese nodules is shown in Table 1. The manganese nodules in Table 1 were those collected from the sea-bed of off-shore California.
TABLE 1 ______________________________________ Composition of Manganese Nodules Mn Fe Co Ni Cu Residues ______________________________________ 16.31% 11.8% 0.32% 0.53% 0.25% 70.8 ______________________________________
It is known that the valuable metals are dispersed and incorporated in a Mn and Fe oxide mineral phase (matrix) of manganese nodules and therefore, in the utilization of the valuable metals, it is necessary to destroy the matrix to thereby change the form of Ni, Co and Cu into a form capable of being easily extracted. As the treatment therefor, a number of processes have been proposed. They include, for example, (1) direct leaching with an acid or an alkali under severe conditions, and (2) oxidative roasting or reductive roasting followed by leaching with an alkali.
However, none of these conventional processes are advantageous in the recent situation where the energy cost rises rapidly, because they require considerably severe treatment conditions and consequently expensive equipment and because a large quantity of energy must be supplied externally. Moreover, the extraction percentage of valuable metals have not been high, and particularly the extraction of cobalt has not been satisfactory in any conventional process.
A method for the treatment of manganese nodules has been previously known in which valuable metals in manganese nodules are rendered solvent-soluble by forming an electric cell using manganese nodules as a cathode active material component and discharging the cell. According to this method, manganese nodules need not be treated with a reducing agent, because the Mn matrix and the valuable metals dispersed and accommodated therein can automatically be reduced very easily by the electric discharge, and subsequent severe treatment conditions such as elevated temperatures can also be eliminated, because they are already rendered readily soluble in a solvent such as a dilute acid under very mild conditions. Moreover, the valuable metals in manganese nodules can be recovered by way of recovering energy from the manganese nodules rather than by supplying a large quantity of energy externally.
If this method is employed, however, a large quantity of Mn is also rendered solvent-soluble by the electric discharge, so that the extraction percentage of Mn would exceed 60% in the extraction treatment after the electric discharge, and it is no longer feasible to separate Mn from each of other valuable metals.